Ribonucleic acid (RNA) from separated parts of cattle lenses, i.e. epithelial cells, cortical and nuclear fibres was prepared by phenol treatment. The heterogeneity of ribonucleic acid was studied by sucrose density-gradient ultracentrifugation and methylated albumin kieselguhr column chromatography. Due to the differentiation of epithelial cells to lens fibres, marked decrease in relative amounts of high molecular weight RNA and an increase of low molecular weight RNA were observed by both methods. Similar distribution was shown when RNA heterogeneity of chick lenses during ontogenesis, starting from the embryonic stage, was studied. The method of methylated albumin chromatography proved to be more advantageous when the features of low molecular weight RNA, present in large quantity in lens, are studied.